This invention relates to methods for performing nuclear magnetic resonance (NMR) studies. More specifically, this invention relates to improved methods for advantageously combining several acquired NMR images of the same region of a sample object to improve the signal-to-noise ratio of the resulting image.
Various techniques are known for producing NMR images which may be of medical diagnostic value. In general, imaging techniques require that the object or person studied be placed in a homogeneous magnetic field and subjected to radio frequency (RF) excitation and magnetic field gradient pulses. The RF pulses excite nuclear spins in the object region of interest to produce NMR signals. Magnetic field gradient pulses function to localize the NMR signal to a specific region within the object and to encode spatial information. Imaging techniques differ in the sequence and types of RF and gradient pulses employed. A particular combination of pulses is generally referred to as a "pulse sequence." For a given pulse sequence, the time intervals between pulses are called the pulse sequence timing parameters.
The observed NMR signals depend in known ways on the distribution of nuclear spin parameters, such as nuclear spin density (M.sub.o), spin lattice relaxation time (T.sub.1), and spin-spin relaxation time (T.sub.2). The contribution of each such nuclear spin parameter to the observed signal varies with the particular imaging technique (pulse sequence) and choice of timing parameters.
For example, in the spin-echo imaging technique one or more 180.degree. time reversal RF pulses are used to generate multiple spin-echo signals for each RF excitation pulse. In the case of multiple spin-echo signals, the signals originate from the same nuclear spins in the excited region, but occur at different echo times TE.sub.i, i=1, . . . 4, etc., where i denotes the first, second, etc., spin-echo signal. In the course of a complete scan, sufficient data is collected to reconstruct an image corresponding to a spin-echo signal at each time TE.sub.i. Such images are known as acquired multiple spin-echo NMR images. The images are not identical since the contrast between various tissues changes at different TE.sub.i due to differential spin-spin (T.sub.2) decay. In addition, the overall signal level decreases causing the images to become darker as TE.sub.i increases. Hence, TE is the timing parameter for the spin-echo pulse sequence that affects the contribution of T.sub.2 to the NMR signal and ultimately image contrast.
In medical NMR imaging one reason to acquire multiple spin-echo images is that various pathology, such as, for example, edema, is better visualized in images sensitive to longer TE times, whereas an associated tumor may be better imaged at shorter echo times. Also, since it is known that at certain combinations of T.sub.1 and T.sub.2, some tissues or pathology can have zero contrast, the multiple echo scan reduces risk of not visualizing a structure or pathology. A further reason that such multiple spin-echo images are routinely acquired is that the scan time is not substantially increased by the acquisition of more than one spin-echo signal. This is due to the fact that the time used to acquire the additional spin-echo signals is normally spent waiting for the nuclear spins to return to equilibrium prior to the next excitation step.
One way in which it would be desirable to utilize such multiple spin-echo images is to attempt to, for example, average the images in some way to improve the signal-to-noise ratio (SNR) in the averaged image. It is known, for example, that if images are acquired with the same pulse sequence and timing, then a new image which is the average of the other images will exhibit an improved signal-to-noise ratio. However, although multiple spin-echo signals are acquired of the same region with the same pulse sequence, they do not have the same timing, since each image corresponds to a different spin-echo time. The result in that the differing timing produces images in which the same structures have different contrasts so that simple addition of the images can result in an image which will not be simply interpretable and, hence, not very useful.
A similar difficulty exists interpreting images acquired from single spin-echo sequences wherein the images of the same object region are each acquired for the same value of TE, but for different values of pulse sequence repetition time TR. Each image would have different contrasts due to differential spin-lattice (T.sub.1) recovery. Hence, TR is the timing parameter for the multiple TR pulse sequence that affects the contribution of T.sub.1 to the signal and image contrast. Such images may be acquired for reasons similar to those discussed above for multi-echo images. Different pathology in this case may have best contrast at different values of TR. However, obtaining (e.g., four) images with different values of TR will require significantly more time than acquiring one image, unlike the multi-echo case mentioned above. Again, it is desirable to combine the images acquired at different values of TR so that the resulting image has clearly interpretable features and also exhibits an improvement.
It is therefore an object of the invention to provide a method for meaningfully combining multiple spin-echo images to produce an image having an improved signal-to-noise ratio.
It is another object of the invention to provide a method for meaningfully combining spin-echo images of different pulse sequence repetition times TR to improve the SNR of the resulting image.
It is a further object of the invention to provide a method for meaningfully combining images resulting from pulse sequences which have different T.sub.1 and/or T.sub.2 weighting.